1. Field of the Invention
The present invention relates to a method for the continuous production of esters of thermally unstable acids in essentially only one reaction zone in the liquid phase that is capable of providing high quantities of the esters.
2. Discussion of the Background
Various esters of thermally unstable acids are important intermediate products for the production of other valuable substances. For example, cyanoacetic acid esters are needed for the syntheses of heterocycles. 4-Chlorobutyric acid serves as a starting material for the synthesis of cyclopropane derivatives. A continuous esterification method is desired for the production of the needed technical quantities of these and other esters.
Gas phase esterification methods (see, for example, Ullmann, Encyclopedia of Technical Chemistry, 3rd Edition, 1967, Volume 18, p. 69) cannot be used because of the thermal sensitivity of the acids. DD 127 117 describes a continuous method that works in the liquid phase, wherein the acid is esterified in three successive stages in the presence of a catalyst--in the first stage in a cocurrent flow, in the second stage in a counterflow, and in the third stage, again in a counterflow. This method suffers from the following disadvantages: (1) it requires an additional entrainer for the removal of water generated during reaction, (2) it is expensive with regard to the apparatus used because of its three esterification stages, (3) it requires a considerable measurement and control technical outlay because of the different parameters in the three esterification stages, and (4) it tends to be otherwise troublesome to carry out.
In the production of cyanoacetic acid, the temperatures in the three esterification stages are 75.degree. C. (column 4), 130.degree. (column 1), and 110.degree. C. (column 2), with conversions of 10 to 15%, 70 to 80% and 10 to 15%, respectively. This procedure takes into consideration the fact that cyanoacetic acid already begins to decompose beyond approximately 80.degree. C. Accordingly, the temperature is increased step by step, and the end temperature of 100.degree. is attained in a stage when only a small amount of unesterified cyanoacetic acid remains in the reaction mixture.